It will become obvious if you don't have enough eye relief; your field of view will be constricted or vignetted.

It's also possible to have too much eye relief, causing you to see crescent shaped shadows that seem to move around in your optical field.

So, what exactly is eye relief ? A technical definition is "the distance from the last surface of the lens of an eyepiece to the plane projected behind it where all the light rays of the exit pupil come into focus." This is pretty heavy optical terminology in just one sentence for non-techies!

Here's all you really need to know about eye relief...

Binocular models aren't distinguished for eyeglasses wearers and non-eyeglasses wearers; a given pair accommodates both users through the use of a retractable eye cup (above image) on the eyepiece of a binocular or spotting scope. When the eyecup is twisted down, a larger amount of the eye relief is available for eyeglasses wearers. When the eye cup is twisted up, less of the eye relief is available, which is fine for people who don't wear glasses. The reason glasses wearers need a little more of the total eye relief is because their eyeglasses are in the way so the binocular's focus point can't reach the person's eye.

As a rule, if you wear glasses you'll need at least 15mm of eye relief. However, if your glasses rest further out on your nose, or if you have a strong prescription with thicker lenses, you may need 16mm to 18mm. The good news is that most binoculars today have ample eye relief. Just be mindful that you don't choose a binocular (or spotting scope eyepiece) that has 12mm or 10mm or else you won't be able to wear your eye glasses when viewing through it.